While the ArcGIs 9.4 (later 10) Beta was underway I was made aware that Esri was looking into Ubuntu but at this stage it is not supported, and does not work.

In saying this it does depend on which portion of ArcGIS Server you are looking at. If you are looking at the database portion, then you can install the database on your Ubuntu Server and then install SDE on another machine pointing to your database. At this stage this is the only way Ubuntu can be included in the server operations of ArcGIS Server.

The main problem is as soon as you install any OS to the VirtualBox it will be a non-licensed copy or it will cost as licensed. It would be better to install ArcGIS to a Ubuntu computer itself without anything in between ...

That is certainly possible. In fact I would recommend using a Centos VM and not windows, because you could use paravirtualisation (KVM,Xen) which will offer better performance.
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johanvdwMar 13 '11 at 20:32